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Anomalous Reliability Behaviour of 99.99% and 99.999% Pure Aluminium Wire Bonds under Thermal Cycling
Keywords: Wirebonding, Reliability, Thermal Cycling
In this paper, we report on the reliability of ultrasonically bonded 99.99% (4Ns) and 99.999% (5Ns) pure aluminium wires under different passive thermal cycling ranges, namely -40C to 190C, -60C to 170C, -35C to 145C and -55C to 125C. In contrast to commonly accepted empirical relationships, the rate of bond strength degradation during cycling (determined through shear testing) was found to be more rapid in the wire bonds subjected to lower peak temperature (Tjmax) and lower temperature ranges (∆T) for both wire types. In addition to these unexpected findings, the 4Ns wire bonds were found to be weaker than the 5Ns bonds under the cycling ranges investigated. In this paper, we give details of microstructural characterisation undertaken to shed more light on these results. During thermal cycling, the development of grain size and morphology of the wires was found to be different for the various cycling ranges, and in particular they exhibited different levels of recovery and recrystallisation at the bond interface. These microstructural differences have been attributed to annealing effects occurring in the wire during the high temperature phase of cycling which remove some of the damage accumulated during the low temperature phase. Furthermore, differences in crack propagation rate and cracking mode were noted for the two wire types. These differences are also thought to be related to effect of time at temperature and wire purity on diffusion kinetics during cycling. We conclude that annealing effects which occur during thermal cycling can reduce damage accumulation and can influence wire bond reliability.
P. A. Agyakwa, Research Fellow
The University of Nottingham
Nottinghamshire NG7 2RD,
UK


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